Locomotive auxiliary power FAQ

Q: Why is 72 volts used in locomotives for
lights?

A: The auxiliary battery on a locomotive consists of 8 x 8 volt
batteries. So the nominal voltage is 64 volts, the open circuit voltage is 68.8
volts, and the float charging voltage is 72 volts. So 72 is about as high as
the locomotive's power bus can get. It is really a 64V battery. Sometimes the
"boilerplate" will list 74V, but it is still a 64V battery.

Q: What voltage are the train's lights?

A: The
train's lights are rated at 75 volts, which is equivalent to 14V for a 12 volt
battery.

Q: Can an engineer use the 72VDC to charge his
cell phone?

A: Yes, if you find the right charger. Some of the AC cell
phone chargers are very flexible on their input voltage. Switchmode power
supplies immediately rectify the AC, so they will run on higher voltage DC. The
problem is that 72V is pretty low, since even rectified 90VAC is 127VDC. But
some of these chargers will still put out a regulated 5V to charge the cell
phone when powered with 72V. But of course not all of them will. There are DC
converters that are rated for 72V input and work very well for powering 12V and
24V equipment in a locomotive.

Q: Train light bulbs take the same socket as household bulbs.
Will they work at home?

A: They use the same socket, but their filaments
will vanish in a puff of smoke when used at 110VAC.

Q: Why 72V?

A: The higher the voltage, the lower the
resistive losses. Using the same wire you can transmit 4 times as much power
with only twice the voltage. But as we all know, when the voltage is high
enough it can give you a nasty shock. 72V is at the ragged edge of being in
shock territory. But because it won't kill you it is safer to run around the
cab, and doesn't require the safety codes of higher voltage.

Q: Has the locomotive voltage always been 72V?

A: No, it
seems that many steam engines had 32 volt dynamos. In that era passenger cars
were also 32V and often had their own batteries and generators. Some of these
generators were axle-driven. Cars with AC tended to be 110V because of the
convenience of using 110VAC air conditioners.